text:
Why are 737-200 engines more susceptible to separation?
It seems the 737-200 had an issue with separation of engines from the wings. Considering that the engines are mounted completely under the wings, there should be more attaching points than a modern jet engine the question is why is this aircraft so prone to this sort of incident? Is it something to do with the airflow and the forces associated to it? Or is it purely a manufacturing defect?
Source:
https://www.flightglobal.com/news/articles/nationwide-737-engine-loss-inquiry-finds-fatigue-crack-in-mount-220017/
Snippet from the article:

Fatigue failure of aft cone bolts has been attributed to a number of
  engine-separation incidents involving 737-200s – among them a US Air
  aircraft at Deptford in December 1987, a United Airlines jet at
  Chicago in January 1989, and a Delta Air Lines aircraft at Dallas in
  January 1992. All three lost their right-hand engines on take-off.

Source? Link? Anything?

In general, jet engines are mounted under wings using only a few attachment points- Boeing uses cone pins in 737-100s and 200s (these were changed into fuse pins from 737-300s).
Older 737s with cone pins (the SA 737 is a 200) has a history of failure and almost all the the cases being blamed on the fatigue and manufacturing defect of the aft bolt (which usually resulted in over-stressing and failure of the two front bolts). I've just listed some of the incidents and the probable causes (from NTSB):
Incident: January 03, 1986 involving BOEING 737-2H4, registration: N86SW

Probable Cause: ...EXAMINATION OF THE CONE BOLT REVEALED THAT IT FAILED AS A RESULT OF FATIGUE, MOST PROBABLY DUE TO IMPROPER INSTALLATION OF THE BOLT, SPECIFICALLY, THAT IT WAS UNDER TORQUED WHEN THE OPERATOR RE-INSTALLED THE ENG.

Incident: December 05, 1987 involving BOEING 737-2B7, registration: N319AU

Probable Cause:  ... THE AFT MOUNT CONE BOLT FOR THE #2 ENG HAD FAILED FM FATIGUE THRU THE THREAD RELIEF UNDERCUT RADIUS. FATIGUE CRACKS HAD INITIATED ON DIAMETRICALLY OPPOSITE SIDES OF THE RADIUS.

Incident: January 20, 1989 involving BOEING 737-201, registration: N242US

Probable Cause: PREVIOUS DAMAGE TO THE AFT CONE (ENGINE MOUNTING) BOLT, WHICH RESULTED IN MISMATCHED SURFACES BETWEEN THE BOLT AND ISOLATION MOUNT, LOSS OF TORQUE DURING CYCLIC LOADING OF THE MOUNTING BOLT, AND SUBSEQUENT FATIGUE FAILURE OF THE BOLT

Incident: January 07, 1992 involving BOEING 737-232, registration: N322DL

Probable Cause: THE FAILURE OF THE AFT CONE BOLT AS RESULT OF PREEXISTING FATIGUE CRACKING DUE TO IMPROPER MAINTENANCE, AND THE FAILURE OF THE SECONDARY SUPPORT STRUCTURE AS A RESULT OF LOADS THAT EXCEEDED THE CAPACITY OF THE ATTACHING HARDWARE AND THE CRUSHABLE HONEYCOMB CORE.

The incident involving the Nationwide 737-230 was also traced back to fatigue of the engine bolt:

The right-hand engine separated from the aircraft due to the failure of the aft cone bolt as a result of a pre-existing fatigue crack which was most likely caused by incorrect installation of the cone bolt.

No investigation has pointed to any problem with the airflow or associated forces.

In a related note, 747s, which use fuse pins also had a number of incidents/accidents caused due to failure of the pins and Boeing changed the design quite a few times.

When Boeing first designed the 737, their focus was to reduce weight by having a short and simplified landing gear. This affected how the JT8Ds were fitted to the wings (directly underneath, no pylons) and introduced the design and associated issues that @aeroalias has explained so well.
It was only when the 737-300 was being designed to have the larger (read quieter and more efficient) CFM engines that the engine position was changed (from directly under wing to slightly forward of it), and allowed Boeing the opportunity to do away with the cone pins.


text:
Which aviation flight computer is Spock using aboard the Enterprise (as a prop)?
In the recent Undecided with Matt Ferrell video Why the Future of AI & Computers Will Be Analog after about 05:44, the Original Star Trek TV show's Mr. Spock is shown holding an analog flight computer. I don't know the episode, but by the looks of the open electronics racks in the background on the left, this seems to be an emergency situation and the device is a backup measure.
Asked separately on Science Fiction & Fantasy: In what TOS episode did Spock use a prop known as an analog flight computer, and why did he need to turn to a "handheld device" in this case?
Is it possible from the screen capture here, or from other sources, to determine which model of flight computer was used to film this scene?
Note that Star Trek the Original Series or "TOS" was filmed between about 1966 and 1969.

In general, it was never that analog computing  didn’t do its job — quite the opposite. Pilots still use flight computers, a form of slide  rule, to perform calculations by hand,  no juice necessary.

It's certainly an E6B. Are you looking for the exact manufacturer and manufacturer P/N? If so, that might be hard to find, but the internet is the right place to ask that question!

@KennSebesta If someone recognizes that or knows of an online "flight computer museum" (similar to the Transistor Museum) they may be able to pin it down. That would be wonderful! I'd probably add a bounty to that answer.

There are various models at the Smithsonian's National Air and Space Museum.

It looks just like the ubiquitous E6B.

Yes it does have the right form factor. It would be great to see a photo of the back side. Perhaps the one in the question is an older, vintage version - the yellow plastic ring...?

If Spock uses it then it's good enough for me!

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Why is the sound quality of onboard public address so bad?
While not a frequent flyer, I did fly common airliners quite a lot, and each time the (very) low sound quality of onboard public adress system has always struck me.
We all know technology makes it possible to have a "HiFi" sound quality almost everywhere, so why don't major manufacturers (Boeing and Airbus) provide better sound quality systems?
While this is of course not mandatory to make an airplane fly, it is related to passenger safety. Better sound quality would allow crew instructions to be better understood during the pre-takeoff briefing and in the event of an emergency.
I have flown with several airlines, and there are already enough other factors that make it difficult to understand what's being said on PA:

On board noise: air conditioning, running engines, neighbors talking, kids crying, ...
Strong or difficult english accent when either the crew or the passenger are not english-native speakers.
Crew speaking really fast.

The airplane companies can't do much on the above items, but they can improve the technical aspects. Of course, I do understand that this would require increasing weight (loudspeakers are heavy...) thus reducing commercial load.
Side question: How do US/EU regulations handle this point? Do they have any specs on sound quality requirements?
Note: I haven't tried recent planes such as A380, so maybe it has been improved.

On the point about a bad accent, I've been on a few different European airlines. They usually speak live in their native language (German, Norwegian, French, etc) and then play an English MP3 using the FAP. Or if the crew is English, vice versa.

Crews speaking really fast: I notice this, and find it very annoying. Crews must have made the same announcement at least thousands of times, but passengers need to hear it!

United Kingdom civil aviation authority's specification on PAS

Thanks @anshabhi, so this UK regulation indeeds requires a comprehension rate that needs to be achieved.

Probably for the same reason the salads are so bad.

@TylerDurden: They have salads?  I must be flying on the wrong planes...

For the regulatory portion of this question.

14 CFR 121.318
No person may operate an airplane with a seating capacity of more than 19 passengers unless it is equipped with a public address system which—
(a) Is capable of operation independent of the crewmember interphone system required by§ 121.319, except for handsets, headsets, microphones, selector switches, and signaling devices;
(b) Is approved in accordance with§ 21.305 of this chapter;
(c) Is accessible for immediate use from each of two flight crewmember stations in the pilot compartment;
(d) For each required floor-level passenger emergency exit which has an adjacent flight attendant seat, has a microphone which is readily accessible to the seated flight attendant, except that one microphone may serve more than one exit, provided the proximity of the exits allows unassisted verbal communication between seated flight attendants;
(e) Is capable of operation within 10 seconds by a flight attendant at each of those stations in the passenger compartment from which its use is accessible;
(f) Is audible at all passenger seats, lavatories, and flight attendant seats and work stations; and
(g) For transport category airplanes manufactured on or after November 27, 1990, meets the requirements of§ 25.1423 of this chapter.
[Doc. No. 24995, 54 FR 43926, Oct. 27, 1989]
14 CFR 25.1423
A public address system required by this chapter must—
(a) Be powerable when the aircraft is in flight or stopped on the ground, after the shutdown or failure of all engines and auxiliary power units, or the disconnection or failure of all power sources dependent on their continued operation, for—
(1) A time duration of at least 10 minutes, including an aggregate time duration of at least 5 minutes of announcements made by flight and cabin crewmembers, considering all other loads which may remain powered by the same source when all other power sources are inoperative; and
(2) An additional time duration in its standby state appropriate or required for any other loads that are powered by the same source and that are essential to safety of flight or required during emergency conditions.
(b) Be capable of operation within 3 seconds from the time a microphone is removed from its stowage.
(c) Be intelligible at all passenger seats, lavatories, and flight attendant seats and work stations.
(d) Be designed so that no unused, unstowed microphone will render the system inoperative.
(e) Be capable of functioning independently of any required crewmember interphone system.
(f) Be accessible for immediate use from each of two flight crewmember stations in the pilot compartment.
(g) For each required floor-level passenger emergency exit which has an adjacent flight attendant seat, have a microphone which is readily accessible to the seated flight attendant, except that one microphone may serve more than one exit, provided the proximity of the exits allows unassisted verbal communication between seated flight attendants.
[Doc. No. 26003, 58 FR 45229, Aug. 26, 1993, as amended by Amdt.
  25-115, 69 FR 40527, July 2, 2004]

There are systems in place to increase volume in certain instances, eg, on the 767 both engines running gives a 6 dB increase and O2 masks dropped gives an additional 3 dB gain in volume.

A good percentage of the flights I've been on failed (c), and I would expect that virtually all commercial aircraft would fail that requirement when ambient noise levels are high, which would often be the case in emergencies when a PA system is needed most. I think it just underscores the validity of the OP's question.

Not disputing the awfulness of the typical PA system. From DC-9 to B777 they're all pretty bad. There are systems in place to increase volume in certain instances, eg, on the 767 both engines running gives a 6 dB increase and O2 masks dropped gives an additional 3 dB gain in volume. This probably really just means louder intelligibility.

The PA (Public Address) system is composed by a band-pass filter between a range of frequencies $f \in [f_i , f_f]$
The quality of the sound is affected because the voice frequencies may exceed this range and thus, the band-pass filter rejects these peaks, that are exceeding the filter range.
Remember how a band-pass filter works:

The frequencies below $f_i$ are not allowed nor the frequencies above $f_f$. Nevertheless, when the frequencies are outside of the bandwidth range (determined by the $\Delta f=f_2-f_1$ at which the gain has a value of $\mathrm{gain_{max}}-3$ $\mathrm{dB}$), the quality of the sound gets worse and therefore, from the cabin we are listening to a dimmed sound of the flight crew voice.
Some of the newer aircraft PA systems are done by using DSP (Digital Signal Processing) systems, also within the limited range, but the quality of the sound is more improved.

References: Dimov Stojce Ilcev, "Global Aeronautical Communications, Navigation and Surveillance (CNS)", June, 2013.

Looks like a nice answer but any chance you could add a source or reference to back this up? :)

Are you sure about that ? You have any source ? Because what you describe is the standard "telephone" filtering (weather it is POTS or VoIp), and I don't see in any way how keeping a wider bandwith would interfere with radio comm. These are not at all the same freequencies!

The RF spectrum starts at frequencies of 3 kHz  I never heard of any radio communication device using frequencies below 100 kHz... moreover in airliners ! Remember that VHF airband is around 100 MHz and long-range HF is around 2MHz.

This seems to be completely wrong. There is no way band-filtering sound can prevent interference with RF signals. The sound is transmitted over a narrow RF band anyway, the frequency distribution of the sound has nothing to do with the frequency of the RF signal.

Sorry if it caused any inconvenience. It seems it was wrong the RF frequency interference part of my answer. What must be correct is the part that I found on the cited book, that is, the mode of operation of the aircraft PA system: either a bandpass filter or a notch filter. I edited my answer only keeping the referenced information.

To me this is more of a 'how' than a 'why'. Public address systems are well-understood technology and it shouldn't be difficult to build ones in planes that work well, even under adverse sound conditions. Such sound systems would, however, be more expensive. I strongly suspect that is the 'why'.

The microphones used in aviation headsets are simple electret microphones, with high vocal clarity but low response at high frequencies.  They provide a quality similar to standard telephone service.  Since the pilots use these microphones to speak on the internal PA, there's no reason to install a high fidelity PA fed by a low fidelity microphone.
The PA's purpose is clear transmission of information, same as the radio.  High voice quality is a very low priority, and it's also difficult to control without a custom microphone setup for each individual pilot.
It's also worth noting that your basic assumption is that a higher fidelity system would improve the passengers' ability to understand the crew.  This is not actually true.  Low-bandwidth systems such as telephone operate on the principle that speech can be understood as well (and sometimes better) with its high frequency components removed.  This offers the additional advantage of requiring lower channel capacity per user of the system (in both telephone and radio comms).
A high quality audio address system, such as that used by a radio DJ, requires a quiet environment, a high-quality, highly-sensitive microphone, and a speaker with a consistent voice, who maintains a consistent distance from the microphone.  None of these are practical in an aircraft cockpit.
A slightly lower-level system, such as that used by a speaker on a stage, would use a microphone on an earpiece in a similar location to that of the pilot's microphone.  However these are condenser microphones, which are non-directional and highly sensitive.  Such a microphone would be overwhelmed by background noise in an aircraft cockpit.
In both the radio setting and that of the speaker on a stage, there is also a dedicated sound engineer who is monitoring the level of the speaker and making adjustments to the microphone output.  Equalizers, compressors, and reverb are also employed among other effects to improve the quality of the speaker's voice.

You don't need recording studio quality equipment to improve a PA system. PA systems on planes should equal or exceed the quality and intelligibility of PA systems frequently encountered in shopping areas, sports arenas, schools, etc. At the very least, a volume boost is needed. I've been on many flights where the PA was unintelligible because the volume was simply too low.

Thanks for your answer. While I agree with most (not all) of your statements, I understand your answer as "can't really do better". I can't agree with that, some improvements -can/need to- be done.

simple electret microphones, with [...] low response at high frequencies: This is simply not true, a 1$ micro capsule has natively a very good bandwidth. But headset manufacturers do probably cut it down to avoid larsen effect.

@Carey Gregory - the intelligibility of a PA is not closely related to its quality.  Telephones, for example, are highly intelligible despite their very low quality.  Volume is a separate issue, and one that is directly controlled by the flight crew.  If it's too low then maybe they aren't doing their jobs correctly.  Volume, however, is something highly subject to personal preference.  What is too quiet for some is inevitably too loud for others.

@kebs - My answer is not "can't really do better", it's "can't practically do better."  The PA is a system that is made to work without thought or attention from the pilots.  Improving it would require them to devote attention to its operation, which is simply not worth it.  Also WRT Carey's comments of other PAs, the aircraft PA is similar or better quality than that in a grocery store or school, it's just in a louder environment.  Comparison to a sports arena is irrelevant for obvious reasons.

@kebs - I stand corrected on the microphone's frequency response.  You are probably correct that they reduce it further down the line, though.

Are you seriously telling me it's impractical to train flight crews to speak intelligibly on a PA? The same argument applied to radios would be met with laughter. And if someone with good hearing such as myself often finds PA volumes too low then I think it's a safe bet they often are too low, which shouldn't even be possible. I believe they most certainly can practically do better.

I am not "seriously telling you" anything WRT training flight crews.  However I did mention that the volume of the PA system is adjustable.  The pilot's can't hear what the volume is, so I would guess the flight attendants normally adjust it.  Like anything adjustable, it will never make everyone happy.  (Something I know from years of sound mixing.  What's too loud to some will be too quiet to others, always.)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the incidence angle of a propeller in the terminology of the Wright Brothers and how did they obtain it?
What is the angle of incidence of a propeller in the terminology of the Wright Brothers and how did they obtain it?
In Feb. 1903 (see Screw Test 1903, page 8), using a 0.58 hp engine, the Wright brothers tested a propeller characterized by:
Diameter, D = 8.5 ft
Pitch = 15 deg
Surface, $S = 2\ ft^2$
RPM = 245
They measured what appears to be the static thrust and got:
Thrust = 18.75 lbf
Based on the assumption that the propeller can be modeled by a wing (Pitch = 15 deg, Surface = $2\ ft^2$, Lift = 18.75 lbf) having its entire area concentrated in a single point (called  the Center of Pressure, C.P.) traveling at RPM = 245 on a circle with the diameter 0.824 x D, $V_{wing} = RPM \times \pi \times 0.824 \times D$, the Wright brothers evaluated its Drag and from Lift, Drag and Pitch using trigonometric relations they obtained the Normal (perpendicular to the chord) and Axial or Tangential (along the chord) components of Lift + Drag (the resultant force) and some angles related to them.
Now comes something I do not quite understand.
The two brothers calculated a reference force defined as:
Total Normal = $k \times S \times V_{wing}^2$ = 24.8 lbf,
where
$V_{wing} = RPM \times \pi \times 0.824 \times D = 61\ \mathrm{mph}$
$k =$ Smeaton’s coefficient $= 0.0033\ \mathrm{lbf} \cdot \mathrm{ft}^{-2} \cdot \mathrm{mph}^{-2}$
Further, the Wrights divided the Lift by the Total Normal
$\frac{\mathrm{Lift}}{\mathrm{Total\ Normal}} = 0.756$
and from this 0.756 they drew the conclusion that the angle of incidence is 7¼ deg. The question is why? What is this angle of incidence and how can it be calculated?
Update
The correspondence between the percentage 0.756 and the angle of incidence, 7¼, comes likely from a table like this (which is for a slightly different wing curvature):
Percentage of air pressure at various angles of incidence

However, as the propeller did not move, the incidence angle of its equivalent wing appears to be identical with the pitch of the same wing and they are not. Why?

If a wing (a small helicopter blade) rotates about the axis of a stationary motor, how do you define the pitch and incidence angle? This is the question. It appears that Pitch = angle (chord, plane of rotation), AoA = angle(chord, relative wind) = angle (chord, plane of rotation) as the relative wind speed is in the plane of rotation. It seems that, for the Feb 1903 test, Pitch = AoA also called incidence.

Possible W. Wright, based on the speed of the air blown back by the propeller, believed he was in the equivalent case of a propeller moving forward, a situation when AoA is different from Pitch and this is the reason he did not see a contradiction when he obtained an angle of incidence different from Pitch.

According to various scholars, the Wright brothers were referring to "angle of attack" when they used the term "incidence." They actually published an article by the same title, so the fact is well-established.
References include the following:

In Wilbur's time, the "angle of incidence" was what we now call the "angle of attack" -- the angle at which the wing of an airplane meets the wind (wright-brothers.org).

and

In the Wright's own words: "The lift is thus balanced against the normal pressure on the resistance surfaces ... Therefore, the lift ... at the given angle of incidence (angle of attack) is to the pressure on a square plane of equal area at 90° as the sine of the (...)
(wright.edu)

The angle of attack of a propeller is defined in various ways, a summary of which is here.

The angle at which this air (relative wind) strikes the propeller blade is its angle of attack.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are “Kennedy Steve’s” various jokes allowed in ATC communications?
Forgive me if there is an obvious answer to this, but I was just listening to this “Kennedy Steve” playlist, and he seems to joke a lot more than most ground controllers I’ve listened to.
I have three questions about this.

Is it legal for air traffic controllers to joke on the air at all, and definitely this much?
Are Ground controllers held to the same rules regarding communication as Tower, Approach/Departure, or Center?
Is it more serious for a pilot to start joking on ATC, or for a controller to, in terms of consequences?

Related question about ATC phraseology

@fooot that's sort of related, but that's asking more about whether apologizing is OK, my question is whether joking around for no real reason is OK

On a relatively slow frequency as long as you aren't issuing conflicting or confusing instructions, this isn't expressly forbidden, however FAA would like you to keep things "professional". A little joking around can usually be heard, whether it be back-handed comments (like telling a 747 "caution, wake turbulence Cessna 150") or just a little friendly banter about a local sports game (happens frequently here at KGRB).

Just a related discussion about the importance of clarity and brevity in communication, not saying it all applies exactly here.

@fooot ah OK, I thought you were suggesting it was a duplicate. ;)

He sounds like Neil deGrass Tyson.

"Is it more serious to start joking..." Not sure if it's a positive or negative expectation :)

(US-based answer)
Yes, it's legal. Yes, everyone - including pilots - is held to the same standard of professionalism. And there are no real consequences for anyone (usually). The simple fact is that we're all human beings and humor, slang and jokes are all part of how we communicate and even handle stress.
Legally, the FAA doesn't regulate radio communication, the FCC does. Of course, the FAA has aviation regulations and guidelines that apply, but they're very general (as they should be). Using standard phraseology is strongly encouraged at all times for clearer, quicker communication. The ATC orders 2-4-14 say:

Use the words or phrases in radiotelephone and interphone
  communication as contained in the P/CG or, within areas where
  Controller Pilot Data Link Communications (CPDLC) is in use, the
  phraseology contained in the applicable CPDLC message set.

In other words, stick to standard phrasing. But there's nothing in there about avoiding humor or jokes.
As for different controllers, they all follow the same standards for radio use no matter where or on which frequency they're working. In my experience, the quieter the frequency the more likely it is that someone - controller or pilot - will joke or even have a brief conversation. I was once taxiing a C172 to the far end of a 12,000ft runway - which takes some time - and the tower controller asked me a few short questions about my accent, where I was from etc. But I was the only aircraft on his frequency at the time and I'm sure he wouldn't have done it if it would have impacted operations or safety in any way.
Which brings me to your final point. In normal circumstances, no one cares about this stuff at all because a) it has no significant impact on anything, and b) it makes life a little more interesting for everyone. It's practically impossible to fly cross-country for more than half an hour without hearing someone make a joke, chat with someone else (briefly), or even complain about something. And again, in normal circumstances that's completely fine: no one expects pilots and ATC to act like robots with a limited vocabulary.
But, if a joke or non-standard phraseology causes offence or a misunderstanding or - in the worst case - an incident of some kind then the FAA or NTSB could be interested. This column from NASA's ASRS team has some examples of reports of problems causes by non-standard phraseology, including this one:

"I called for clearance to St. Louis as follows: 'Clearance delivery,
  company ident, ATIS info, federal aid to St. Louis.' Federal aid was
  meant to mean FAA clearance in a joking fashion. The Controller
  misinterpreted this to mean that we were being hijacked and called the
  FBI and airport police...I used no 'standard' phraseology to indicate
  nor was it my intent to indicate we had a hijacking...I will use
  absolutely standard phraseology in the future..." (# 248982)

And this incident got quite a lot of attention, even outside the aviation press. I don't think it makes sense to say that it's 'more serious' for ATC to make jokes than pilots because every incident is different anyway.
All in all, joking on the radio is like joking in any professional context: if it doesn't disrupt your work or other people's work, if you're not insulting people or wasting their time, and if you're not saying anything stupid or dangerous then no one really minds.

It's worth noting that the FAA can write up a controller for non-standard phraseology (and would almost certainly do so if it lead to a deviation, loss of separation, or some other incident). Also folks like Kennedy Steve and Boston John, as long-serving full-performance controllers with extensive experience have a sense of when joking around won't cause trouble, and in fact use standard phraseology more than they joke around. If a new controller were to cut up on the frequency like this they'd probably get a "talking-to" from the facility manager...

Wow, to be honest I don't even see how saying go around could be interpreted as a joke in any context. It's like a pilot calling mayday as a ''joke''...

The important thing to realise here is that "Kennedy Steve" is an experienced veteran controller, so he knows exactly what is appropriate and when.  When there's heavy congestion (not rare at JFK) or an emergency in progress, the jokes stop and everything is business; once the crisis is past, a little bit of levity diffuses the tension.

"Qantas 12, follow the single engine Cessna, caution propwash."

JFK is also a major international airport, so some of the pilots have only a limited command of English.  Kennedy Steve does his best to accommodate these as well, switching to more standard phraseology and clearer diction.  Even then, he sometimes has to repeat himself.
English is also the standard language of the sea.  Nevertheless, radio conversation does occur in local languages as well.  This is a famous example from the Gulf of Finland; after the initial Mayday call in English, much of the conversation is in Finnish and Swedish.


text:
Why does the Cessna 172S Skyhawk use only two blade fixed pitched prop?
Surely it would get more thrust from a three blade variable pitch propeller. From what I've read it's all about the area swept and also should be far enough apart to not cause too much interference. Besides having a constant speed prop would allow for a wider range of optimum operation.

You're not wrong, but a fixed pitch two-blade prop has a pretty big advantage: it's cheap. Cheap to buy, cheap to install, cheap to maintain, cheap to repair. 172s are predominantly trainers in FBO fleets, and cheap is the winner. The efficiencies of a better prop are lost when you're just doing pattern work and short cross countries.
Also, when you're not climbing, a two-bladed prop is actually going to be more efficient, not less as it's creating less drag. According to the linked Hartzell FAQ:

Depending on the combination of [engine power, operating RPM for the propeller, diameter limitations, design requirements, etc] a 2-blade propeller may be most efficient, but as power increases additional blades are generally required to efficiently utilize the increased power.

This means that on a 160 horsepower engine like the 172 is rocking, you aren't going to see particularly tangible benefits from increasing the number of blades. Going to a constant speed prop would, on the other hand, give you greater utilization of power across the entire flight envelope.
For example...
An overhauled McCauley fixed pitch 2-blade propeller that fits a Cessna 172 is $2,100 on ebay. It has no moving parts (I mean, aside from itself) and is relatively easy to balance.
A (non-overhauled?) Hartzell 3-blade constant speed prop off a Piper PA-46-350 Malibu Mirage is $15,000 on ebay with no logs, which is usually a bad sign. It's not a direct comparison to, say, the prop on a C182, but it's probably in the ballpark. It's harder to balance, and requires more maintenance to make sure the constant speed mechanism is up to snuff.
I was only able to find prices on ebay, which isn't the most scientific comparison ever, but it's a start.

You make an extremely valid point good sir. From what you're saying price and the reputation of the C172 as being a training aircraft are the biggest factors as to why it isn't a constant speed prop. Which makes alot of sense now that i think about it. Cheers

@egid: A constant speed prop would add significant weight to an engine that (in this case) sports a 160 horsepower engine.  The weight is about as far forward as possible.  This creates a Weight and a balance problem for your typical training load of two adults in front as the sole occupants.  The extra power that a constant speed prop would achieve just isn't worth the cost, complexity, and W&B limitations.  Looking at the existing fleet, constant speed props don't become common until the 0-360 engine and larger.

@SkipMiller that's certainly a good point. But it's not what the OP was asking, so I didn't bring it up!

I can't improve on @egid's answer on efficiency or cost, but another point on constant speed units is related to 172s predominantly being trainers: fixed pitch props are easier to fly.
My first few lessons in the circuit were busy enough just getting the altitudes, positioning, radio calls and checks completed correctly. When I did differences training for variable pitch a few years later it suddenly became busy again, simply from trying to remember what the RPM and MP settings I was supposed to be using at each stage (generally 5 different combinations per circuit).
It's not much harder, admittedly, but just having one dimension (the black throttle knob goes forward for more power or back for less power) was easier on my already over-taxed student brain than the two-dimensional variable pitch setup (black knob for power, blue for RPM; with blue going forward both when you want more power (takeoff) and when you're decreasing power (short final, in case you need to go-around)).

For a given horsepower two blades are more efficient than three, three more than four, and so on.
There are a few reasons to have more than two blades:
-the engine generates so much power that a two blade prop would need to be so long that it would contact the ground.
-the engine generates so much power that a two blade prop would need to be so long that the velocity of the blade tips would exceed the speed of sound.
-the application of the aircraft requires acceleration and climbing ability over top speed.  More blades for a given power equate to slower top speeds but better takeoff acceleration and rates of climb.
-in the case of the V-tail Bonanza, the added weight of the three blade moves the CG forward making loading easier.
The most efficient prop for a given hp is a single blade (it exists).  Plenty of examples in Google images.
The cost and manufacturing/maintenance aspect are other considerations and have been addressed in other answers.

This is not a hypothetical question since Cessna did manufacture versions of the 172 that included constant speed props. These included:

The FR172, aka "The Reims Rocket", produced in France from the late 1960s to the mid 1970s.
The R172K Hawk XP, produced in both France and the US from 1977 to 1981.
The Cessna Cutlass 172RG, produced from 1980 through 1986.

However, none of these aircraft proved especially popular with the general public. Even with higher horsepower engines, the common perception was that the increases in speed and useful load were too small to justify the additional cost and complexity.
When Cessna started producing propeller aircraft again in 1996, they focused on their most popular models, such as the standard Cessna 172 and the 182.
If you wanted an an aircraft with significantly better performance than the 172, you were expected to buy the Cessna 182, an aircraft which was very popular with the public. The Cessna 182T has a 3-bladed constant speed prop.
So, to answer your question, yes those kinds of improvements would have and did increase the performance of the Cessna 172. But they were not popular and, thus, were not worth the effort.


text:
What is "required navigation performance" or "RNP"?
I understand that this is some sort of on-board navigation monitoring and alerting...what is it used in? Do the Garmin G500s, G1000s, or the G430s use it? Where can you find it? I know nothing about this and I am trying to figure out what it is.

RNP and RNAV are implementations of the overall idea of performance-based navigation. The principle is to increase navigation accuracy to enable more efficient uses of the airspace (more direct and optimized routes). This can be summarized this way.

See also What is a SAAAR instrument approach? where the answer from DeltaLima includes a description of RNAV and RNP and a link to a very interesting presentation of RNP by FAA.

RNP (Required Navigation Performance) is a standard which avionics manufacturers may choose to design/certify their products for.  It guarantees a particular level of navigation performance that routes and procedures can be designed around, and not have to worry about how the navigation is being conducted, but only that it will meet requirements.
A particular product may be certified to various levels such as RNP10, RNP5, RNP2, RNP1, RNP0.3, etc. (accurate to within 10NM, 5NM, 2NM, 1NM, 0.3NM, etc.).
To see if a particular unit is certified for RNP, and to what level, you would need to look in the aircraft flight manual / supplement and see what it says (note that this is installation specific too, as it involves antennas and "full system" performance, not just the capability of one piece of the puzzle).
The FAA has an advisory circular caled AC 90-101A - Approval Guidance for Required Navigation Performance (RNP) Procedures with Authorization Required (AR) Including Change 1 if you want more technical information on it.


text:
Why do the engines on a 737 have a flattened lower surface?
Inspired by this answer.
The engines on a 737 are not perfectly circular when viewed from the front. The bottom of them are flattened a bit.

Why is that? Is it for aerodynamic or ground clearance reasons?


text:
Which airplane types never crashed?
Is there an airplane that never crashed?
Or more specifically, without any crash history on any database. By crash, let's say any accident / incident which resulted in damage to the airplane and/or injuries to the people on board.
Not including military airplanes, only commercial or general aviation aircraft. Obviously I'm asking about a plane that had multiple flights and maybe is even still flying, not an experimental built that only flew a few times.

There definitely are, mostly planes with a small user base and of recent introduction. I.E. think of a newly introduced high end glider (fully certified, GA) with only a few produced . It may take a while before someone has a bad landing in it but I do not think this would be a satisfactory answer.  You may want to be more precise in you question, adding e.g minimum number of aircraft built, definition of crash (hull loss? fatal?), if in commercial use, years of service, etc

Perhaps some clarification is needed for the purpose of the topic. What would mean "crash" ? Hull loss, belly landing, plane skidding off the runway etc. ?

This website list 5 aircraft types that have never had a fatal accident (Aibus A340, A380, Boeing 717, 747-8, 787) http://www.airsafe.com/events/models/rate_mod.htm.

Here's a very recent report that quotes 10 aircraft that have never had a fatal accident (in addition to the above, Bombardier C series, CRJ regional jet, Boeing 737 MAX, and Airbus A350, A320 neo): https://www.pressreader.com/australia/the-west-australian/20180104/281732679869583 You would have to take this list and find which/if any had never had any non fatal event.

A full answer to this is going to be a very, very long list and one that is constantly changing. Airliners crash so rarely that new types often go well more than a decade with nothing that I would consider to be a 'crash.' The 777 went 13 years before its first crash and nearly 2 decades before its first fatal one, despite probably having the 2nd most number of total flight hours for a widebody behind only the 747 by that point. Additionally, the GA fleet has a lot of types that don't have a lot of frames. However, I would expect all GA types with lots of flight hours to have had crashes.

@Penguin If we're treating separate generations as their own aircraft, the 2nd gen 777 would also be on the list (-300ER and -200LR.) All 3 of the fatal accidents (and the 1 non-fatal one) have been on the 1st gen (-200ER.) And, of course, the 3rd gen 777 (777X) will soon join the list.

@reirab. I was just quoting the aircraft types listed at those websites, to save you the trouble of having to read them, but exactly the same thought crossed my mind that you state in your comment! It's perhaps not a fair / consistent classification of aircraft "type". Cheers. (PS. That list of 10 was put together by Boeing).

"By crash, lets say any accident / incident which resulted in damage to the airplane and/or injuries to the people on board." -- that is an extremely broad definition of crash. There are plenty of incidents that cause severe damage to the aircraft but still end with a relatively normal landing and passengers able to walk away from the plane.

And the 777 hull losses didn't have anything to do with the airplane.  It has a perfect record if you set aside things that are totally not Boeing's fault.

What if two passengers get in a fist fight? That's definitely an incident and results in injury. Do terrorist attacks count?

@JörgWMittag and clear air turbulence, where the plane drops 10,000 feet and people not wearing seat belts break all sorts of bones.

@reirab.  Why is the question too broad?? If anything, it's too narrow. The definition of "crash" is quite inclusive, and so the number of types that haven't "crashed" is actually very limited. It's certainly not a long long list that is constantly changing. It's a list of 5 aircraft, but is really  3 aircraft because 2 of them really aren't sufficiently different from other versions that have crashed. And if the question is too broad, how come it has been answered, and upvoted? Can you please unlock this question. I would like to add additional information. Regards.

@Penguin The question asks about all commercial and general aviation aircraft, not just the airliners that have been discussed, so far. That's a lot of types. And it will be constantly changing as new types are created and existing ones have their first 'crashes.' SE questions are just not a good format to maintain such lists, which is why list questions are typically closed across the network. Also, even if this question had a static answer, it would really need to use one of the formally accepted definitions in order to get accurate data, rather than just "damaged the aircraft."

@Penguin Also, note the couple of comments above yours, which point out some of the ambiguities in the definition used here that would make it really hard to find reliable and complete data, let alone for every commercial and GA type ever made. Also, note the first couple of comments, which raise similar concerns.

@Penguin It's not that the question is necessary 'bad' or that the question or its answers are uninteresting; it's just that SE's format isn't really a good fit for this sort of question. A Wikipedia list would probably be a better fit, but a more formal definition (for which data is available) would need to be specified.

@reirab. OK, thanks for taking the time to give such a comprehensive reply. The decision does make more sense now, though the crux of the issue IMHO is the inclusion of "general aviation aircraft", as while I initially thought it was very hard to answer, for commercial airliners the Aviation Safety Network website actually has the necessary data in a pretty quick/easy to find format, and new commercial aircraft aren't introduced very often, or crash often(though incidents are more common), so that list is more static. It's just a pity, it looks a reasonably popular question. Regards

@Penguin If the OP wants to reduce the scope to airliners and specify an incident scope that's a little more specific and easier to find comprehensive data on (say, hull losses or passenger deaths,) then perhaps this could be reopened. A mere bird strike would meet the current definition of 'causes damage,' but those happen daily. Answers would still suffer from becoming stale every now and then, but not nearly as often. Also, the existing information only seems to cover Western models and not, say, Soviet bloc or Chinese ones, though there should be information on those out there somewhere.

As mentioned in this answer: there are seven commercial aircraft types with zero hull loss accidents:

B717
CRJ700, CRJ900, and CRJ1000
A380
B787
B747-8
A350
C-series
A320Neo

Only the first two types have logged more than 1 million departures. The remaining ones are relatively recent types and are still working on statistical significance; especially the long haul aircraft fleet takes a long time to accumulate 1 million departures at 1-2 flights per day per aircraft.

Hi @Koyovis.  Just want to clarify - in the linked answer it says there are 6 types with zero accidents "working on their 1 million departure milestone". Here you state 2 have achieved that. Is there a source for actual number of departures (or best current estimate)? Also, has the A340 had a hull loss but not a fatal accident? I'm not sure I'd consider the A380 "new", some airframes are 10 years old. See  http://www.airfleets.net/listing/a380-1.htm Regards.

The A380 has been in commercial service for more than ten years -- I wouldn't call that "new". However, there aren't a huge number of them (about 220 in service, according to Wikipedia) and they fly long-haul routes so don't make many departures.

@Penguin Yes, the A340 has had 5 hull losses, but no fatal accidents (though one of them did result in 12 serious injuries and dozens of minor ones after a runway overrun.) 3 of the hull losses were during landings and the other 2 were on the ground (one destroyed by hostile actions, the other by failing to chock or brake the aircraft during an engine run-up test, resulting in a collision with a concrete wall.)

don't forget the 737MAX. If you count the 320NEO as a separate type, that one is as well :)

Yes indeed, the graph predates first delivery of a MAX.

Hi @Penguin apologies, missed your comment. The only reference I could find is the Boeing document linked to in the other answer. Must be in the low millions I reckon, the 717 fleet is quite small.

Sadly, the B737 MAX had to be removed from the list due to JT610.

A comprehensive answer to your question would be very difficult.  The Honda Jet has never crashed.  The Boeing 787 has never crashed.  The A380 has never crashed, but both appear in safety databases based on incidents that have occurred: engine emergencies on the A380 and electrical fires on the 787.

Which is quite reassuring, since it shows that the A380 can handle engine trouble safely, and an electrical fire on a 787 isn't fatal.


text:
Do the military really use GA planes as target practice?
In the September issue of the British Microlight Aircraft Association (BMAA) magazine, someone wrote in to say that whilst flying their C42, two Typhoon fighter jets passed them (one either side) a few times at high speed, at the same altitude, before flying away.
They said, when they contacted the nearest tower frequency, that they were told the military sometimes use GA as target practice (obviously they're not actually doing any 'firing').
The writer did provide some Flightradar24 screenshots to prove the incident had happened, but I wonder if ATC really told them that.
Do the military use GA for target practice?
This was all in UK uncontrolled airspace by the way.

Strange "target practice". Intercepting an aircraft (e.g. lack of radio contacts) or shooting an enemy plane are done in different ways (never on both side).

If there is a link to the actual article in the magazine, this might be better suited to [skeptics.se] (skeptics requires a notable source, hence the need for an actual article rather than just "I saw....")

What is BMAA magazine?

I won't answer, as I have nothing authoritative, but many years ago I back seated on an RAF Nimrod sortie over the North Sea and they absolutely used civilian vessels as targets. Not for weapons, but as contacts to close in on, to ID using Sonar etc. It wouldn't surprise me to hear that Fast Jets do similar, though again, I doubt this is 'target practice' in any form, but rather RADAR intercepts, tracking etc though there is an obvious level of increased danger vs maritime patrol and boats

@tedder42 British Microlight Aircraft Association. I can't link the article as it is not digital, and I don't want to post a picture of the print without their permission

@Cloud I think what you are asking for are "intercepts" instead of "target pratice". An intercept is to guide your aircraft such that it meets up with another aircraft. Target practice is to shoot at something for practice. I very much hope that the jets in question did not shoot down GA aircraft :D.
Because of this difference, you also get answer for aerial gunnery practice instead of intercepts...

Is it not clear that however much they might choose to deny the very idea, still
any air force which didn't use GA planes for target practice would be shirking its responsibilities?

@RobbieGoodwin, that's a pretty broad and unsubstantiated accusation.  Air Forces have plenty of training aids at their disposal that don't require the passive and unknowing cooperation of civilian aircraft.  That's not the same as saying they may illuminate and even lock onto GA aircraft occasionally for practice, but to imply that not doing so is "shirking its responsibilities" is pretty extreme.

@MichaelHall That's not at all 'broad' and it doesn't need substantiation, except to people with no real interest in military operations and training.
You and anyone else who doubts that is welcome to explain at least why you think your country's military should be denied the most realistic training they could get.
Will you try that or explain why not, please?

@RobbieGoodwin, I have 20 years in military aviation in the USMC and USN. I wasn't an F16 driver, but I can confidently say that running radar intercepts on passive Cessna 172s is not "the most realistic training they could get", and not doing so does not impair our warfighting capabilities in the least. We honed our skills fighting very capable military trained aggressors, and I trust that the USAF still does the same thing today.  Will you please explain how you have arrived at your opinions on effective military air combat training?

@RobbieGoodwin, you are coming across as unhinged.  "Denying a wider accusation"?
What does that even mean?!

Cessna 172s are a ubiquitous GA aircraft, I just chose something at random.  Would it make a difference to my point if I had not specified actual type aircraft?  And my experience started 40 years ago, and "understanding" some time prior.  But to your point, so what?!

"who cares whether radar intercepts on any passive craft is '… most realistic training"  Well, it seemed like you did in the first comment that triggered this...

@MichaelHall Who doubts that denying something irrelevant about, eg, Cessna 172s is a standard tactic for avoiding a wider accusation by trying to force the focus onto a more-specific case? In like mode, who cares whether radar intercepts on any passive craft are '… most realistic…'?
Who doubts failing to use every method available amounts to negligence, despite your personal views?
My experience of military aviation started not 20 but 50 years ago and so what? For reasons outside your ken, my understanding of military aviation started not 50 but more than 60 years ago and so what?

Please, Michael, either back off or go to Chat.

Let us continue this discussion in chat.

As for the US, they do practice intercepts on GA planes and they aren't supposed to. A F-16 was involved in a hull-loss crash due to perceived loss of control during a low-speed interception, and the mishap investigation report states:

The MFL [Mishap Flight Lead] and MP [Mishap Pilot] Failed to identify/correct risky or unsafe practices by intercepting a non-participating GA aircraft
Through the course of interviews with the mishap unit, there was a lack of consensus regarding whether the intercept of a non-participating, uncoordinated GA aircraft operating outside a MOA is a permissible training event. However, a thorough understanding of the existing training rules makes it clear that any simulated target of interest (TOI) must be non-maneuvering, or only maneuver with full coordination between the TOI, the controlling agency, and the ACA interceptors.

Do you have a source for the claim that "they aren't supposed to" perform practice intercepts on GA planes?

@MichaelHall Did you bother to read the bottom 75% of the answer?

I did.  It doesn't say what you are claiming.  In fact, "there was a lack of consensus regarding whether the intercept of a non-participating, uncoordinated GA aircraft operating outside a MOA is a permissible training event." says almost the opposite.

@MichaelHall Read the very next sentence.

I have. That pertains to maneuvering. Without a more complete discussion on training rules, one could say that intercepts on non-participating aircraft are permitted as long as they are not maneuvering. From that quote alone you cannot say "they aren't supposed to". That's the statement I am asking you to justify.

Neither does the part in bold you referenced validate your claim that "they aren't supposed to". Failing to properly identify a risk is not the same thing as violating a policy or regulation.

@MichaelHall "failing to correct unsafe practices" implies violation pretty clearly to me, as does the quoted note on what the training rules make clear. Pointing out that the unit lacked consensus on that is obviously pointing out a failing, not suggesting the opposite. And non-participating aircraft can't possibly be "non-maneuvering or only maneuver with full coordination"... it could maneuver at any time. As the report points out. Read it if you doubt what user71659 is saying, but the answer is pretty clear to me as is.

@mbrig, I get your point, but poorly and unsafely executing some action that is otherwise permissible is not necessarily a violation of that which you are permitted to do safely. In crude terms, "failing to correct unsafe practices" simply means that the pilot f@#$ed it up. (I spent my career reading safety investigation reports, this is standard verbiage for pilot error...) Good point too on the lack of consensus. Which highlights that if the active duty members of the squadron were unsure it is doubtful that any of us internet randos will know the answer for certain.

Regarding "maneuvering" this term has a specific meaning in the context of ACM, or Air Combat Maneuvering. Yes, a non-cooperating civilian target could turn at any time, but that isn't what the training rules are referencing. (I inserted the term maneuver "defensively" in another comment string to emphasize this, and was lambasted...)

I can’t speak for what Great Britain or some other government does, or what you mean by ‘target practice’.  In the United States, for aerial gunnery practice, we employ a number of unmanned air vehicles as targets, most notably, the QF-16, which is a retired F-16 fighter converted for this purpose.  I have never heard of light general aviation aircraft being used as targets for aerial gunnery exercises.
General aviation aircraft are sometimes used to simulate threats where the military assets interact with them, but do not fire upon them.  One example are joint exercises with the civil air patrol.  CAP will use Cessna 172 and 182 aircraft to intentionally violate practice TFR areas, allowing for military fighters to fly simulated intercepts.  And it comes in handy, as NORAD reports hundreds of TFR violations every year; mostly people with drones, but there are occasions where some ding dong doesn’t read the NOTAMs and flies his Piper Cherokee into a presidential TFR or similar event.  They are generally  intercepted and handed over to law-enforcement on the ground.
It is possible as well, that military aircraft may track civilian aircraft using fire control radars in exercises for practice, similar to what was described above.  I do know the military has also used civilian ground structures as practice ‘targets’ in simulated air strikes. I remember reading stories from Colonel Tony Tolin, who used to command the 37th TFW (F-117As) out of Tonopah, NV.  He said that, during the 1980s, their aircraft would fly specific missions at night, targeting specific buildings or structures discussed in preflight briefs as a simulated strike mission in preparation for war.  I remember one case where Mr. Tolin stated that  their target is a specific cabin with a dock on the north shore of Lake Tahoe.  The only thing missing was that the jets never dropped the bombs (nor did they carry any on these flights), and for additional safety, never lased civilian buildings as well.

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

In the airspace I used to work, we had several Military Operations Areas (MOAs) that extended from 500 feet above ground level up to, but not including, FL180. When the military used the airspace, all IFR aircraft were routed out of/around the airspace to allow them to practice air combat maneuvering.
Nothing precluded VFR traffic from flying through an active MOA. Sometimes you would see the fighters line up on an untracked VFR target and fly pretty darn close to the VFR aircraft. We're they "targeting" the VFR? I don't know for sure, but it sure seemed like it on radar.

Fascinating. Did you ever discipline them for this?

Honestly, other than violating separation (for GAs) I really dont see much of an issue with interrogating, targeting, even locking a GA aircraft to practice use of the targeting systems. Its literally just different patterns of radar sweeps. Flying too close to a non-participant? Yeah, not cool from a safety standpoint. But acquiring, and locking from 10km away? Please, do explain to me the terrible harm being done other than keeping our fighter crews trained on how to use the hundreds of millions of dollars systems their service branch strapped to their butts....

@TankR. - would you be cool with someone pointing a loaded gun into your face to do some scope checks, and they pinky promise they wont shoot by accident? The militaries around the world have killed enough civilians through sheer stupidity that I would never be comfortable with nonsense like that.

@TankR. To simply avoid the negligible chance of firing on a civillian target by mistake, they are never used for target practise. Period.

@Davor, military aircraft do not normally carry live ammo into MOAs.  They very rarely carry any live ammo anywhere in CONUS.  If they do, it would be in a tightly controlled live fires exercise in a restricted area.  Not a MOA with civilian traffic around.  Even during coordinated large scale exercises in a MOA, (such as Red Flag) coordinators will call "knock it off" if there is an interloper.  The loaded gun example you suggested simply doesn't happen.

@MichaelHall - the loaded gun is proximity itself. And also it's hilarious how many times non-live ammo turns out to be quite lively.

@Davor, I don't think it's hilarious at all, and would challenge you to find a single instance in the last 75 years of a US military aircraft accidently shooting down a civilian airplane during a training exercise.  (Alec Baldwin's recent mishap and others like it are irrelevant in this context if that is what you are hinting at...)  P.S.  What do you mean by "proximity itself"?

@Davor, I am very familiar with that incident.  The causal factors are completely irrelevant to this discussion.

@Davor In the friendliest way I can put this... I am convinced yall need to take a deeper look into the process required to actually launch a piece of locked on ordinance from a combat aircraft. Its not as simple as 'wait for tone, OOPS! My finger slipped...' I am not aware at the moment, though I'm sure a quick google will net an answer, but its possible the radar system of the aircraft (not on the missiles, if equiped) could be used to find, track, and lock without activating any potentially loaded, and armed, ordinance. Its literally just different types of radar sweeping patterns.

@Jpe61 see above. Normal radar is a regular interval sweep (regular repeating spike of RF energy). The track phase is a more active sweep that keeps the returned signal mostly in the aperture of the scan area while, scanning around the returned signal to better work out the signals bearing, distance, speed, etc (a very fast series of possibly irregular RF spikes as the area around the craft is scanned). In the lock phase the singal is centered wavering slightly around to detect any deviation in its flight path (constant RF singal). The () bits are how tracking and lock of enemies is detected.

@TankR.  Military planes do track civillian trafic with their radars, that's for sure. What counts as "targerting" here may be up for discussion. I understood the question such that "target practice" means actually targetting as in acquiring as a target for weapons.

They're the same systems, other than the missiles that have radar systems onboard. Even then the aircraft radar feeds the ordinance internal radar data so it knows where to point its radar, then informs the pilot it has the target. But to loose a missile still isnt a 'oops, my finger slipped' kind of affair. Heck, if thats the worry, there are platforms that can detect, track, and target by multiple systems (land, sea, and air), patched through an EWACS to a command center, which could then order a missile fire, and guided to the target using this massively interconnected data network...future


text:
Why is the range of transponder digits 0 to 7 rather than 0 to 9?
As a controller on IVAO I sometimes assign codes like 0168 and find out later that the code isn’t valid. For me, the range 0-9 seems more logical. Why do we have to use octal numbers and not decimal?

Possible duplicate of Why do the transponders introduce codes instead of using aircraft registration numbers?

@fooot, In which universe could this possibly be considered a duplicate?

The four-digit transponder code that is entered by a pilot is an octal number rather than a decimal number, and in the octal numbering system, only the digits 0-7 are valid.
As far as why, internally it's actually a 12-bit binary number and octal works really well since can be used as a "shortcut" for entering groups of three binary digits at a time (000 - 111, which is 0-7 in octal).
If it were designed today, with the additional computing power and software complexity available, it would be possible to use a decimal number even if it is stored as binary internally, just so that it is more familiar.  At the time that the system was created though, changing the dials directly set the code in hardware and doing the conversion would have been much more complex / difficult / expensive.
In short, it was convenient for the engineers and manufacturers, and just an oddity for the end users, so they left it that way.

In addition, even though nowadays it would be possible to use decimal encoding, with dials that allow setting 0000...4095, you would need to prevent setting 4096...4999 (or 9999 if you didn't limit the first dial to 0...4). Using octal dials, you cannot select outside the valid range of 0000...7777.

As a side note; This is the very same reason IPv4 adresses contain 4 0-255 numbers (2^8 options), MAC adresses are written down as a set of hexadecimal pairs (2^8 options in range 00 to FF). Both are used because of very easy conversion from/to binary system. Since user doesnt do any math with such addresses, there is no drawback at all.

"If it were designed today..." they would use 24 bit hexadecimal codes :)

@J.Hougaard Not necessarily, as Crowley mentioned in his comment, IP addresses are commonly referred to in decimal even though they are stored as 24 bit codes...  :)

@Lnafziger I was referring to mode-S

I want to upvote this answer, but its value is exactly 2^6 and I just can't destroy this beautiful number and dare everone who upvotes it!

Fascinating.  This also explains why the emergency codes start with 111 binary (7 decimal)...

Octal representation is a cheap way to deal with small binary numbers
Modes A and C transponders send their ID using a train of pulses of unmodulated carrier, which can be seen as a sequence of bits. Today, such transponders use 12 pulses (mode S transponders use a 24-bit address which is different):

Source
Pulses are labeled A4-A2-A1, B4-B2-B1, C4-C2-C1 and D4-D2-D1 (interleaved in the transmission). They are contained within two framing pulses for transmitter-receiver synchronization purpose, and there is an additional pulse X in the middle, always at 0 today, but used in the glory days as custom data by manufacturers (e.g. to report to ATC the position of a retractable landing gear).
The transponder must be told the value of each bit in some way. Today it wouldn't be a problem to enter the ID as a decimal number (between 0 and 4095), but when the transponder was invented, decades before the arrival of inexpensive programmable chips, a reliable converter was a costly device.
On the other hand, it was not an option to just provide the 12 values using On/Off switches:

Sample of miniature switches, source
Obviously mistakes would have been numerous, either during vocal transmission of the ID, or when entering the ID.
The simplest solution broadly used when dealing with bits is to group them, and associate a single symbol to each possible combination in the group.
It would be good to use groups with 10 combinations, we already have symbols "0" to "9" to represent them. Unfortunately when grouping bits by two, we obtain groups with 4 combinations each. If we group by three bits, the groups have 8 combinations. By four, the groups have 16 combinations, etc.
Because we are dealing with a number expressed in radix 2, the number of symbols to be used is always a power of two, never a power of 10. So engineers had no choice at the time, than to forget the decimal numbers and use an odd number of combinations. As an example, let's group by 2 bits and use Greek letters for the required 4 symbols:

We would be poor designers to produce this (except for a remake of Airplane!), but you've got the principle. To keep the system simple, we can use decimal number symbols if we need at most 10 symbols, and complete with letters if we need more. This is exactly what is done when spelling numbers in octal (groups of 3 bits) and hexadecimal radices (groups of 4 bits).
As explained in other excellent answers, the first transponders had only 6 bits for their ID, so octal was the sensible choice, and IDs were presented as 2 octal figures, e.g. "75" to represent "111 101". When the ID length was extended, the octal system remained in use.
In case your are interested by the details of how octal is used, I propose details below.

Technical specification: a 12-bit binary number and 4096 values
The transponder radio signal for the ID transmission is made of 12 pulses (modes A/C) that can be low or high, this allows 4096 ($2^{\small {12}}$) combinations.
As the pulses have only two states, low or high, we can refer them directly as a 12-bit binary number between 0 and 4095. Let's illustrate with some values:

We can add as many 0 on the left as we want (e.g. to have 12 bits), like we do in decimal: 25 = 025 = 00025.
Octal and hexadecimal representations of binary numbers
For convenience, we usually replace groups of bits by some equivalent to reduce the number of signs used. It's common to use groups of 3 and 4 bits:

Octal: A group of 3 bits can generate 8 combinations, we can easily replace this group by a single figure in radix 8 to cover these combinations. Radix 8 is named "octal" numbering.
Hexadecimal: Similarly a group of 4 bits generates 16 combinations (twice the number of combinations for a group of 3, as the two values of the additional bit can be associated with all the 3-bit combinations. We can replace the 4 bits of a group by a figure in radix 16. Such system is named "hexadecimal" numbering.

Back to our examples, replacing the bit groups by their equivalents in radix 8 and 16:

Select which radix to use
What have we got so far? We started with a 12-bit number, grouped bits either by 3 or 4, obtained the octal value (0000 to 7777) and hexadecimal value (000 to fff), two representations of the same binary number.
Which one is better in this case? The answer has been provided in other excellent posts: At the beginning there were only 6 pulses allocated for the ID. Logically, they were grouped by 3, and octal was chosen.
If there had been 7 pulses, probably the hexadecimal representation would have been selected, with values from 00 to 7f.
From radix to electromechanical switch
It is easy to design a wheel switch (thumbwheel) with 8 octal symbols that manages 3 actual switches to generate 3 on/off (low/high) electrical signals:

Source
We use the angular position of the wheel to close or open the three contacts. There are many designs to do this, one is to use brushes rotating over a disc with printed conductive tracks:

Source
The switches provides directly the binary group. By assembling four switches, we could provide the transponder the "squawk" code, that is the binary configuration for the ID pulses.
It's as easy to use 3 hexadecimal wheels with 4 contacts.
Why not 10 symbols?
We cannot group bits of a binary number into groups that can have exactly 10 combinations. This property is limited to power of 2 radix (4, 8, 16, 32...)
It was possible later, with the introduction of integrated circuits (as simple as memories) to convert quite easily decimal to binary. The 74S484 has been one such ICs. It could handle two inputs representing decimal values ("binary-coded decimal", BCD) and convert them in pure binary. The converter for 4 decimal figures (0 to 4095) would be designed like this, using 5 of these circuits:

As you see, it's more simple to use only 8 symbols ($2 ^{\small {3}}$), or 16 ($2 ^{\small {4}}$), and more generally a power of two.
It's legitimate to think we could also use a logic based on simple Boolean gates (NAND, XOR...) to manage the conversion from decimal to binary instead of a memory. This is not apparent at first sight, but it would actually requires a huge quantity of gates.
In order to be convinced, I have done the exercise for only a single bit (out of 12) and for only two decimal wheels out of 4. I removed the details of the exercise, but they can be seen in the edit history. Here is the result:

The minimization allowed to reduce the number of terms (hence gates) to 10. Still, this is only for one bit and two wheels. Sometimes it just doesn't worth the additional difficulty, specially at a time there was no ICs, to stick to decimal.

Engineer much? :) Excellent explanation!

Why on earth would you make a redesign now using complicated 1970s technology like that, when all you need is a simple keypad and a microcontroller - only $10 for both, compared with at least $50 for the parts to build this museum piece? That's assuming you can even source the parts -  the 74S484 chip is long out of production, though there are more modern equivalents.

@alephzero: This is an educational answer, it needs to provide the concepts, not the state of the art. BTW a "simple keypad and micro-controller" are more expensive than you think in the aviation world, you have to make them certified. This is not a world where you can phone Apple to tell them you have a problem.

This is a much better answer than Lnafziger's.

@TylerDurden I believe that it depends on the technical level of the reader, and this answer will be better for a more technical person (like us) while my answer is better suited for less technical types. When I answer a question, I use the complexity appropriate to the original question, and the wording of his question made it seem like he didn't need this level of detail. Good info for others though, and I certainly up-voted this answer to show appreciation for the info and effort that he put into it! :)

@TylerDurden, "better" is a value judgement that requires a goal to compare against.  The goal is always subjective.  Please define "better" according to what criteria.  If the criteria is "fewer words" then this answer is objectively not better.

Short Answer
Entering squawk codes in octal, or base-8 where allowable digits are 0 through 7, makes the human-machine interface between pilot and transponder as simple as possible.
Background
Mode A and Mode C compatible transponders use 12-bit identity or beacon codes. Each bit has two options, high or low, which works out to a total of 212 = 4,096 representable (i.e., that fit within 12 bits) identifiers, ranging from 0 to 4,095 in base-10, written 409510. Now, not all representable indentifiers are valid. For example, 75008 is reserved for specific situations, but that is not for the circuitry to worry about.
Quantities that fit exactly within some number of bits do not line up cleanly with powers of ten. Consider a hypothetical transponder that accepts four decimal digits 0000 through 9999 or 104 = 10,000 possible inputs. The pilot is more likely to handjam a bogus squawk than a representable one. This “simpler” interface now has an error condition that it must deal with: representable and unrepresentable inputs. If the pilot types or spins the dials to anything above 4095, how will the transponder reply? Should it blink an error light that the pilot may miss? How should it reply to interrogation while in a bad state? Silently clip the beacon code to all bits high (111 111 111 1112 = 409510 = 77778)? Stop replying entirely? More modes mean more testing and more expense.
Tradeoffs
‘Ah,’ thinks the clever old-school engineer. ‘No more fancy buttons; we will give the leftmost dial only five positions, 0 through 4.’ But what about the second dial that must be 0 when the first is 4 but can be any digit otherwise?
We have twelve bits to fill. Perhaps the pilot enters the binary directly. “Cessna 123AB, squawk zero-zero-one-zero-one-zero-zero-zero-…” Just imagine the frequency congestion with such long instructions, readbacks, corrections, and confirmations.
Our short-term memory is good for about seven items, give or take two. Twelve binary digits (“bits”) are too unwieldy. Other factors of 12 are 6×2 and 4×3.
Factoring the twelve identity bits into six and two means squawk codes in either two 6-bit quantities (0-63) or six 2-bit (0-3) chunks. Imagine dealing with 64-position dials in the old days. Doing it with decimal buttons gets us back to the problem of unrepresentable codes. Six dials each with 0 to 3 may work, but that makes the dials smaller and forces the pilot to deal near the limit of short-term memory, both making this option error-prone and thus less desirable.
Three groups of 4-bit or hexadecimal quantities could work. Although hex values range from 0 to 1510, a common convention is to use digits 0 through 9 and then A through F for ten to fifteen. In other words, we are down to one digit per place, but in the old days, that would have meant using 16-place dials. Today, that would mean sixteen buttons on the face of the transponder. However, “Approach, Cessna tree-alpha-bravo declaring an emergency and squawking foxtrot-charlie-zero for this charlie foxtrot!” does have some mnemonic value.
The Sweet Spot
Considering it the other way, we have four groups of three bits. In binary, place values are powers of two (not powers of ten like we’re used to thinking in decimal). Working with three bits each, the maximum value for any beacon code digit is
$$(1 \cdot 2^2=4) + (1 \cdot 2^1=2) + (1 \cdot 2^0=1) = 7$$
Here we find several advantages:

Squawk codes have four places — easily manageable in short-term memory.
All digits are familiar 0 through 7, so

Dial inputs have only eight positions or
Eight buttons suffice.

All four-digit pilot inputs are representable in Mode A or Mode C.
The transponder doesn’t have to worry about what to do with bad inputs.

Excellent answer!

The simple answer is that a mode A or mode C transponder allow 12 bits of identification data, and three bits can be described as an octal digit.  Therefore there are four octal digits, of which range between 0000 and 7777.
The question has changed, so I am supplementing this answer.  The convention of using octal numbers for transponder codes dates back to an earlier implementation of transponders which had 6 bits of aircraft identifying data. It was decided (I believe by some white shirted COTR) to go with octal digits. Dealing with computers, octal or hex are easier than decimal.  Later the transponder aircraft identifying field became 12 bits, and the octal convention was maintained.  It is a convention, and it has been maintained.
Supplemental: I remember a transponder which had 6 toggle switches for entering the code, prior to the 12 bit standard. The switches were marked by threes with 4-2-1 underneath them. Back prior to computer literacy of the masses, once can speculate the the selection of digits, rather than for example, hex, entered into consideration as IFF and transponder technology developed. A VFR squawk code could be entered as a 12, which would have most significant 1 switch up, and the least significant 2 switch up. I recall that secondary radar show that the same as 1200, even though it came from a transponder without the 12 bit standard. 77, or all switches up was for emergencies, and 76 and 75 all respectively mapped to modern day 7700, 7600 and 7500 codes.

The 12 bits were set by the system timing and accuracy of pulse generation and processing technology available at the time. Having set 12 bits they could have done 0 to 4095 but that would have required special switches and/or gearing for the display. So to keep things simple they used Octal with 4 simple, 8 position switches.

In computer systems, everything is binary. This is due to the logic circuitry only having 2 states, high and low voltage. Because of this, decimal wouldn't be as simple to convert to binary as octal or hex are.

The decimal-to-binary translation is completely trivial for any computer system (even the cheapest calculator can do it!) - but the first aircraft transponders were used towards the end of WWII, well before there were any computers small enough to fit inside a plane and reliable enough to be useful for this type of application,

These systems were designed many decades ago, and the system remains unchanged for simplicity's sake. But all computers are still binary. And machine code isn't decimal because of this.

and quantum computing is going to blow your mind if you're a fan of counting possible [qu]bit states :)

@mins I've heard of analogue computers, but don't they require near constant tuning to be accurate?

@alex: We know how to have stable voltages, or stable clocks, to a given precision. We can use operational amplifiers, PLL, etc as the base.


text:
In what way are the Concorde's engines considered efficient?
The Concorde is powered by a bunch of Rolls-Royce/Snecma Olympus 593 turbojet engines. Much has been said about how efficient they are at speed, and the fact that they allow long-range cruise at Mach 2 because of how fuel-efficient they are.
If I look at the specifications, one thing immediately obvious is the compression ratio of 15.5 - quite high for a no-bypass turbojet. I'm not sure how this translates into efficiency though.
The thrust-specific fuel consumption, on the other hand, looks dismal. Wikipedia states:

1.195 lb/(lbf·h) (33.8 g/(kN·s)) cruise / 1.39 lb/(lbf·h) (39 g/(kN·s)) sl

Wait, what? The Tumansky R-25, which powered the MiG-21 and was famously fuel-hungry, had a compression ratio of only 9.5, but had a TSFC of

98 kg/(h·kN) (0.96 lb/(h·lbf)) at maximum military power

The General Electric YJ93, which powered the XB-70 and was also designed for long-range supersonic efficiency, had a TSFC of

0.700 lb/(lbf·h) or 19.8 g/(kN·s)

This doesn't seem to make sense: in what way are Concorde's engines any good? Is there something I'm missing?
Finally, I've been attempting to model the Olympus 593 in a flight simulator (Advanced Jet Engine in KSP). With the given compression ratio, though, I couldn't get the fuel efficiency to be this bad: it was around 0.9 SL and 0.85 cruise, and I had to do ridiculous things like using extremely inefficient intakes and nozzles.

KSP is a game, it's not a simulator. 2) the engines were chosen primarily because they were British/French, not because they were the best on the market.

Not an engineer,cant comment on math here. However concorde can travel twice faster than other airliners, so that maybe where efficiency comes from.

@vasin1987 I'm comparing against military aircraft that go similarly fast, not against subsonic high-bypass engines.

@user54609: No fighter from 1960s is capable of sustaining supersonic speed without afterburner. And I don't think any engine had so low TSFC at Mach 2 and with afterburner.

You are missing the speed at which the SFC is given. Comparing static values against data at flight speed is totally misleading. I don't know why @RedGrittyBrick deleted his answer, but it is the best so far. And it is true, the Olympus 593 was indeed the most efficient engine of its time and still holds up well against most of the competition.

Ram air in supersonic flight did improve the efficiency quite a bit, but I have no idea about these numbers. Concorde's efficiency was comically poor when it was on the ground. IIRC those beautiful 593's drank 2 tonnes of fuel for taxiing to the runway. BTW, have you been able to run the numbers on the Tu-144 KN-321 engines? I guess it would be hard because Russia, but I think it would be the best comparison.

You compare SFCs at different speeds. That is like comparing payloads for differently sized aircraft. SFC goes up with speed and, therefore, must be compared at the same speed. The work performed by an engine is thrust times distance, and higher speed means that the same thrust will perform more work per unit of time when the engine moves faster. The moving engine needs to slow down the airflow for combustion to take place, and then needs to accelerate the air by more than it has been slowed down to have positive thrust. Hence, SFC goes up in parallel with speed.
To have a meaningful comparison, we need to define efficiency. There are several, and two are of major importance for air-breathing aircraft engines: Thermal efficiency and propulsive efficiency.
Thermal efficiency
This describes how efficiently the chemical energy in the fuel $Q$ is converted into an impulse change of the air flowing through the engine. Formulated using the mass flow per unit of time $\dot{m}$, the impulse is $\dot{m}\cdot\dfrac{\Delta v^2}{2}$. Using $v_{\infty}$ for the incoming air speed and $v_{\infty} + \Delta v$ for the exit flow speed, the thermal efficiency is $$\eta_{therm} = \frac{\dot{m}\cdot \left((v_{\infty} + \Delta v)^2 - v_{\infty}^2\right)}{2\cdot Q}$$
To achieve good efficiency at high speed, a high $\Delta v$ is helpful. This explains why efficiency drops more over speed for high-bypass ratio engines and especially propellers.
Since the thermal energy in fuel is the same for all engines in your question, because all run on kerosene, and we can assume a similar efficiency of combustion, we can neglect $Q$ in the comparison.
Propulsive efficiency
This describes how well the conversion is performed. Using the same variables as above, propulsive efficiency is $$\eta_{prop} = \frac{v_{\infty}}{v_{\infty} + \frac{\Delta v}{2}}$$
This equation explains the better efficiency of high-bypass ratio engines and propellers at the same speed, because propulsive efficiency is proportional to the inverse of $\Delta v$.
Overall efficiency
This is the product of thermal and propulsive efficiency, and the equation is $$\eta_{total} = \frac{T\cdot v_{\infty}}{Q}$$
where $T = \dot{m}\cdot\Delta v$ denotes the thrust. Conveniently, $\Delta v$ is eliminated in the product, allowing turbojet engines like the Olympus 593 to look much better in comparison to other engines.
Intake efficiency
This answer would be incomplete without a look at the intake of the Concorde. At cruise, it would lift the pressure of the air at the compressor face by a factor of more than six over ambient by efficiently decelerating the flow. The compressor added a compression ratio of 12, so the pressure in the combustion chamber was 80 times higher than ambient. This high pressure makes the engine so efficient, but is also needed to maintain combustion. Remember, ambient pressure in 18 km is just 76 mbar, so the absolute pressure in the combustion chamber at cruise was only 6 bar.
The full answer would be like this: The combination of intake and Olympus 593 at Mach 2.02 had a very good total efficiency, and comparisons with other engines at static conditions are misleading.
The comparison of results from a test stand on the ground would yield a very different picture, however.

Does $\dot{m}$ include the fuel mass being injected into the combustion chamber?

@DrZ214: Normally it is neglected. To be more precise, you can add the fuel mass flow and give it zero as its initial velocity. The difference in thrust, however, is small.

I come from a rocket background so I'm not comfortable neglecting the fuel flow :-) I'm also skeptical that it will have a small change, because dodecane (C12H26) is heavy and O2 and N2 are light. But then again, I would not be surprised if the F-O ratio is less than stoichiometric for temperature reasons. I think I'll post a question on that.

@DrZ214 indeed, jet engines run way lean of stoichiometric (even before including the bypass air) to avoid melting the turbine blades, and to take advantage of unlimited free reaction mass (i.e. the atmosphere).

Wikipedia article on thrust specific fuel consumption actually uses Concorde as example, probably because it was such an extreme case. I probably should edit to make a this a real answer,but since they use your specific question as example I will just quote.

SFC varies with throttle setting, altitude and climate. For jet
engines, flight speed also has a significant effect upon SFC; SFC is
roughly proportional to air speed (actually exhaust velocity), but
speed along the ground is also proportional to air speed. Since work
done is force times distance, mechanical power is force times speed.
Thus, although the nominal SFC is a useful measure of fuel efficiency,
it should be divided by speed to get a way to compare engines that fly
at different speeds.
For example, Concorde cruised at Mach 2.05 with its engines giving an
SFC of 1.195 lb/(lbf·h) (see below); this is equivalent to an SFC of
0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines; it was the world's most efficient jet
engine.[2][3] However, Concorde ultimately has a heavier airframe, and
due to being supersonic is less aerodynamically efficient, i.e., the
lift to drag ratio is far lower. In general the total fuel burn of a
complete aircraft is of far more importance to the customer.

My personal interpretation as a total layperson of this is that the original air speed target was higher than the air speed of Concorde because the issues of supersonic flight were underestimated. Because of those issues the actual Concorde was built for only Mach 2 or so. The engine design was still influenced by that original airspeed target (whatever that was) and as a result had higher exhaust velocity than actually was necessary. This resulted in higher fuel consumption and noise. The reduced range and increased noise in turn limited the routes Concorde could fly and the areas where the supersonic flight could be used. Which made Concorde commercially a "limited" success and made upgrading the engines to ones optimized for the actual speed commercially impractical.
Note that the above is my speculation of the background. The important part is that the exhaust speed of the engines is faster than necessary for the Concorde. This means that despite good thermal and energy efficiency, thrust efficiency is lower than necessary
So yes, the engines were uneconomical and suffered from excessive noise and fuel consumption, but that was due to the airframe and engine being optimal for different speeds. The engines were quite efficient, best thermal efficiently achieved at the time, they just were optimized for the wrong speed that in practice was not achieved.

I was comparing with other Mach 2 engines, though, not with low-speed engines. To be fair though, none of the planes using those engines were able to reach Mach 2 without afterburner, which would significantly increase fuel consumption.

@user54609 Yes, it does. I think the big part here is that Concorde had high exhaust velocity and once you take that into account it was very efficient. A proposed upgrade would have had lower exhaust velocity resulting in less noise and fuel consumption, but since super sonic travel was not a commercial success it wasn't built.

"Yes it does"? What does what? Lol

@user54609 Afterburner, increase fuel consumption significantly. Straight follow up on the comment being replied to.

@user54609 Edited in my interpretation what the quote and data means to the answer below the quote. Not sure if it helps, but at least it looks more like a proper answer now!

The engines were initially designed for Mach 2.2 and redesigned for Concorde, and are stated as getting the 43% efficiency in actual cruise. Concorde was designed to be a Mach 2 turbojet aircraft from the start.

@fooot That is debatable. There is lots of stuff about the issues of supersonic drag being worse than people expected, so I think the actual design process was more complex than is accounted by that. Still you are right that the way I worded that makes it look Concorde is a failed Mach 3 aircraft while what I actually meant was that relative lack of experience with supersonic flight complicated the development process and left some parts badly optimized. I guess I'll reword the answer to focus on that instead of my speculation on the original goals of the project. Thanks.

@fooot Or at least add a note that I don't mean that the actual Concorde was designed for higher speeds...

@fooot Made the edit, hopefully less misleading.

The comparison between different speeds is due to the fact that the SFC figures are per hour, not per kilometer. Concorde @ Mach 2.2 simply flew 2.5 times further per hour than a subsonic plane at Mach 0.85. Thus the "equivalent" SFC figure of 0.5 is just dividing the SFC of 1.195 by the speed ratio's. That's the root cause of the engine efficiency: the faster Concorde covered the same routes in less time. The wikipedia comment correctly notes that the engines are efficient at producing thrust, but the heavy airframe requires a lot of lift and the supersonic wings have a poor lift/drag ratio.

In what way are the Concorde's engines considered efficient?

TSFC/Speed
The Wikipedia article, "Thrust specific fuel consumption", referred to in the question, says

although the nominal SFC is a useful measure of fuel efficiency, it should be divided by speed to get a way to compare engines that fly at different speeds.
For example, Concorde cruised at Mach 2.05 with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this is equivalent to an SFC of 0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines; it was the world's most efficient jet engine.[2][3]

I think what they are saying is perhaps that the amount of thermodynamic work being produced per unit quantity of fuel was high. Concorde was cruising at Mach 2 and had a range of 7000 km. There probably aren't many aircraft around that need to do that. It's engines were producing a lot more work than a typical high-bypass turbofan attached to a widebody jet at Mach 0.85 does.

Thermal Efficiency
They are considered to have high "thermal efficiency" at Mach 2.
They are considered inefficient at lower speeds.

The Rolls-Royce Olympus 593 Mk 610 installed in Concorde STILL remains the most efficient jet engine in the world at Mach 2, as far as thermal efficiency is concerned. This is due to the design of the engine itself of course, but mainly down to the intake, and to a lesser extent the individual nozzle designs. ... (As efficient as the OLY 593 is at Mach 2 and about, at slower speeds it uses fuel like it’s going out of fashion, hence the need for a minimum of low speed flying with Concorde).

From a bunch of Concorde enthusiasts.

Specific Impulse
Another way to measure engine efficiency is specific impulse

Graph by Kashkhan

XB-70 is Mach 3, the question gets a further argument actually

@Trebia: I can't find sources for the YJ93 TSFC in an XB-70 at Mach 3. Wikipedia says the the XB-70 had structural failures at mach 3 and was then limited to Mach 2.5. It also seems to have had problems reaching it's intended range. It's puzzling. Also I can't find if the XB-70 needed afterburners in cruise like the SR-71 did (YJ93 unsurprisingly has 1.8 TSFC with afterburners)

in order YJ93 to have the same efficiency compared with   Olympus 593, that fuel consumption should be set at Mach 1,37 in XB-70. Very likely is higher, but without the actual proper reference, anything is especulation.

Efficiency is an energy thing.
Energy is force times distance.
Differentiate, you get power is force times speed. It turns out you don't even need to know the cruise thrust, you can calculate the efficiency on the TSFC (which is pretty much why it is used).
Although Concorde's engine were generating less force per unit flow, the vehicle was going 2.5 times further each second. If you divide the useful power (thrust times speed) into the power in the fuel (fuel flow times energy per unit mass of fuel) you can calculate the energy efficiency of Concorde's engines.
So let's do that. Here's the basic numbers I found on the web, and I convert them into SI base units:
Cruise speed = 2,124 km/h = 590m/s
TSFC @ cruise = 33.8 g/(kN·s) = 33.8e-6 kg/N.s
Specific energy of aviation fuel = 43.15 MJ/kg
Energy per N m/s = power per Newton = force times speed per Newton = 590 m/s/Newton = 590 Joules per second per Newton
Fuel energy used by engine per N s = 43.15e6 MJ/kg x 33e-6 kg/N.s = 1458 joules per second per Newton
So dividing one by the other, Newtons cancel and we get 40% efficiency.
Bearing in mind that's in an aircraft, which is made as light as possible, for any aircraft, that's amazingly good; better than most electric generating power plants which are bolted to the ground, but some diesel engines can reach over 50% in very, very large ships, and you might exceed that in cars with cast-iron engines which would be far too heavy to be flown.
Let's take a 747-400 at long range cruise with a CF6 engine:
Cruise speed 907km/h = 251 m/s
TSFC 17.1 g/(kN·s) = 17e-6 kg/N.s
So it's generating 251 J/s.N
and burning 17e-6 x 43.15e6J/kg = 733 J/s
I make that 34% engine efficiency.
That's it beating a high bypass turbofan that's used for long distance subsonic cruise. So it's not just that these engines were good for the time; they're still world-class.

I think you are seeing a difference here in where the SFC is being reported. The engines on the Concorde are optimized for supersonic cruise, and SFC is reported there. The fighter is not just optimized for supersonic cruise, so you can't assume the SFC value is for cruise. The YJ93 never saw much use, and the only value I can find for SFC is the one included in the question, but this is probably not at cruise.
Compare the 33.8 g/(kNs) for the Concorde engines to other similar engines, using only values for supersonic cruise. The J-58 (SR-71) at Mach 3.2 was 53.8 g/(kNs) (wet), and the RD-36 (Tu-144) at cruise was 35 g/(kNs).

Doesn't SFC usually decrease with speed though?

I don't think so.

SFC decreases with the exhaust velocity of the engine because the energy contained by the fuel stays the same but the kinetic energy of the exhaust increases with the square of its velocity. If the exhaust velocity stays the same, ie. it was higher than needed for the previous speed, increasing air speed improves SFC. But if gaining the higher air speed requires increasing exhaust velocity SFC goes down. (Bit complex to explain in comment, hope the idea, the link between exhaust velocity, achievable air speed, and SFC was clear.)

@user54609 Thrust- and Power- Specific Fuel Consumption goes down with speed, but that's not the same as SFC.

What is SFC if not a short form of TSFC? Sorry, am a real noob on planes.

Wikipedia: "Thrust specific fuel consumption (TSFC) or sometimes simply specific fuel consumption, SFC, is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output."

I don't think it was the engines alone that allowed supercruise (supersonic cruise without reheat). On Concorde, the engines engaged reheat in order to accelerate in level flight from subsonic to supersonic. One of the main reasons why Concorde had so much more range than the TU144 (which had to stay in reheat to stay supersonic), is the design of the wing. The Concorde wing evolved into a more complex shape during its development (I think between the first prototype and the production Concordes) specifically for this reason. If you look at the Concorde from the front the wing tips kind of sweep downwards. See http://www.concordesst.com/wing.html for more information. Even the great Kelly Johnson didn't do it this way for the SR-71, which stays in reheat the entire time it is supersonic.